Catalysts comprising supported platinum group metals (e.g., platinum, palladium, etc.) are known to have a high capability for removing hydrocarbons from an exhaust gas by oxidation. For example, Japanese Unexamined Patent Publication No. 1976-106691 discloses an exhaust-gas-purifying catalyst that comprises platinum and palladium supported on an alumina carrier. However, in the case of treating an exhaust gas comprising hydrocarbons mainly composed of methane, such as an exhaust gas generated by the combustion of natural gas, the use of such a catalyst still poses the problem of failing to achieve sufficient oxidative decomposition of the hydrocarbons (methane conversion) since methane has high chemical stability.
Furthermore, a combustion exhaust gas generated by the combustion of kerosene, light oil or like petroleum fuels inevitably contains a reaction inhibitor, such as sulfur oxides derived from the sulfur compounds contained in the fuel. Therefore, it is unavoidable that the deposition of the reaction inhibitor onto the surface of the catalyst significantly reduces the activity of the catalyst with time. In addition, in the case of the combustion of fuels derived from natural gas which is fundamentally substantially free of sulfur compounds, as in, for example, the combustion of city gas, which is supplied in many countries, the same problem described above arises, since sulfur compounds are added thereto as odorants.
For example, Lampert et al. reported that in methane oxidation with a palladium catalyst, the presence of sulfur dioxide therein even in such a slight amount as 0.1 ppm causes the catalyst to substantially lose its activity within several hours, and concluded that the presence of sulfur oxides adversely affects the activity of the catalyst (Applied Catalysis B: Environmental, vol. 14: pp. 211-223(1997)).
Also Yamamoto et al. reported the results of an experiment in which hydrocarbons were oxidatively removed from an exhaust gas generated by the combustion of city gas using a catalyst comprising platinum and palladium both supported on alumina. According to this report, the catalytic activity markedly declines after a short time of about 100 hours (Abstracts of the Catalysis Society of Japan Meeting, 1996, published on Sep. 13, 1996).
Further, Japanese Unexamined Patent Publication No. 1996-332392 discloses a catalyst for oxidizing low-concentration hydrocarbons from an exhaust gas containing an excess of oxygen, the catalyst comprising at least 7 g/l of palladium and 3 to 20 g/l of platinum both supported on a honeycomb substrate through an alumina carrier. However, even the use of this catalyst does not achieve sufficient long-term durability and unavoidably entails time-dependent degradation of catalytic activity.
As described above, the main problems of the prior art techniques are that high methane conversion is not obtainable and the methane conversion significantly declines in the presence of sulfur oxides.
In view of these problems, Japanese Unexamined Patent Publication No. 1999-319559 discloses that a catalyst comprising palladium, or palladium and platinum, supported on a zirconia carrier maintains high activity for methane oxidation even in the presence of sulfur oxides. However, this catalyst exhibits low activity for methane oxidation at lower temperatures, particularly below about 400° C. Therefore, in order to obtain high methane conversion according to this technique, a large amount of catalyst must be used, increasing the exhaust gas treatment costs, thus making it impractical.
Japanese Unexamined Patent Publication No. 1991-98644 discloses a method for manufacturing a catalyst for purifying an exhaust gas, wherein, by employing a specific method using citric acid, at least one of platinum and rhodium and at least one of iridium and ruthenium are jointly supported on an inorganic carrier such as activated alumina, etc. According to this publication, iridium and/or ruthenium and platinum and/or rhodium form a solid solution having a high melting point, and therefore the heat resistance of the resulting catalyst can be improved. However, this publication merely discloses that the NOx conversion of the obtained catalyst is improved and it nowhere teaches the oxidative decomposition of m than, which is particularly difficult to decompose among the hydrocarbons contained in exhaust gases.
Japanese Unexamined Patent Publication No. 1999-207182 discloses “a catalyst for catalytic reduction of nitrogen oxides, in which (a) iridium and (b) at least one of indium and gallium are supported on an oxide carrier.” It is disclosed that when nitrogen oxides are reduced using the catalyst disclosed in this publication, various kinds of hydrocarbons in the gas state or liquid state can be used as a reducing agent. Among these hydrocarbons, ethylene, propylene, isobutylene, 1-butene, 2-butene, and like lower alkenes; propane, butane, and like lower alkanes; light oil, kerosene, etc., are recommended. However, this publication also does not disclose a means for oxidative decomposition of methane, which is particularly difficult to decompose among the hydrocarbons contained in exhaust gases.
Japanese Unexamined Patent Publication No. 1995-80315 discloses a de-NOx catalyst for an exhaust gas emitted from a lean-burning engine and a method for treating an exhaust gas emitted from a lean-burning engine. This publication discloses that a catalyst comprising iridium supported on various kinds of inorganic carriers can efficiently remove harmful substances contained in exhaust gases, particularly NOx, even in the lean-burning range. However, this publication also does not indicate that among the various hydrocarbons contained in exhaust gas, methane is particularly difficult to decompose. Therefore, it nowhere teaches the means for efficiently decomposing methane by oxidation.
Accordingly, the main object of the present invention is to provide a catalyst that can exhibit a high activity for methane decomposition even at low temperatures with respect to removing hydrocarbons from a combustion gas containing methane and containing an excess of oxygen, and a method for purifying an exhaust gas using this catalyst.